Biology

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Theories Of Biological Evolution

Lamarck’s theory

Jean Baptiste de Lamarck, was the first to postulate the theory of evolution in his famous book ‘Philosophie Zoologique’ in the year 1809. The two principles of Lamarckian theory are:

(i) The theory of use and disuse – Organs that are used often will increase in size and those that are not used will degenerate. Neck in giraffe is an example of use and absence of limbs in snakes is an example for disuse theory.

(ii) The theory of inheritance of acquired characters – Characters that are developed during the life time of an organism are called acquired characters and these are then inherited.

The main objection to Lamarckism

Lamarck’s “Thory of Acquired characters” was disproved by August Weismann who conducted experiments on mice for twenty generations by cutting their tails and breeding them. All mice born were with tail. Weismann proved his germplasm theory that change in the somatoplasm will not be transferred to the next generation but changes in the germplasm will be inherited.

Neo-Lamarckism

The followers of Lamarck (Neo-Lamarckists) like Cope, Osborn, Packard and Spencer tried to explain Lamarck’s theory on a more scientific basis. They considered that adaptations are universal. Organisms acquire new structures due to their adaptations to the changed environmental conditions. They argued that external conditions stimulate the somatic cells to produce certain ‘secretions’ which reach the sex cells through the blood and bring about variations in the offspring.

Darwin’s theory of Natural Selection

Charles Darwin explained the theory of evolution in his book ‘The Origin of Species by Natural Selection’. During his journey around the Earth, he made extensive observations of plants and animals. He noted a huge variety and remarkable similarities among organisms and their adaptive features to cope up to their environment. He proved that fitest organisms can survive and leave more progenies than the unfit ones through natural selection.

Darwin’s theory was based on several facts, observations and influences. They are:

1. Over production (or) prodigality of production:

All living organisms increase their population in larger number. For example, Salmon fish produces about 28 million eggs during breeding season and if all of them hatch, the seas would be filed with salmon in few generations. Elephant, the slowest breeder that can produce six young ones in its life time can produce 6 million descendants at the end of 750 years in the absence of any check.

2. Struggle for existence:

Organisms struggle for food, space and mate. As these become a limiting factor, competition exists among the members of the population. Darwin denoted struggle for existence in three ways – Intra specific struggle between the same species for food, space and mate. Inter specific struggle with different species for food and space. Struggle with the environment to cope with the climatic variations, flood, earthquakes, drought, etc.,

3. Universal occurrence of variations

No two individuals are alike. There are variations even in identical twins. Even the children born of the same parents differ in colour, height, behavior, etc., The useful variations found in an organism help them to overcome struggle and such variations are passed on to the next generation.

4. Origin of species by Natural Selection

According to Darwin, nature is the most powerful selective force. He compared origin of species by natural selection to a small isolated group. Darwin believed that the struggle for existence resulted in the survival of the fittest. Such organisms become better adapted to the changed environment.

Objections to Darwinism

Some objections raised against Darwinism were –

• Darwin failed to explain the mechanism of variation.
• Darwinism explains the survival of the fittest but not the arrival of the fittest.
• He focused on small fluctuating variations that are mostly non-heritable.
• He did not distinguish between somatic and germinal variations.
• He could not explain the occurrence of vestigial organs, over specialization of some organs like large tusks in extinct mammoths, oversized antlers in the extinct Irish deer, etc.,

Neo Darwinism

Neo Darwinism is the interpretation of Darwinian evolution through Natural Selection as it has been modified since it was proposed. New facts and discoveries about evolution have led to modifications of Darwinism and is supported by Wallace, Heinrich, Haeckel, Weismann and Mendel. This theory emphasizes the change in the frequency of genes in population arises due to mutation, variation, isolation and Natural
selection.

Mutation theory

Hugo de Vries put forth the Mutation theory. Mutations are sudden random changes that occur in an organism that is not heritable. De Vries carried out his experiments in the Evening Primrose plant (Oenothera lamarckiana) and observed variations in them due to mutation.

According to de Vries, sudden and large variations were responsible for the origin of new species whereas Lamarck and Darwin believed in gradual accumulation of all variations as the causative factors in the origin of new species. Hugo de Vries believed that Mutations are random and directionless, but Darwinian variations are small and directional.

Salient features of Mutation Theory

• Mutations or discontinuous variation are transmitted to other generations.
• In naturally breeding populations, mutations occur from time to time.
• There are no intermediate forms, as they are fully fledged.
• They are strictly subjected to natural selection.

Modern synthetic theory

Sewell Wright, Fisher, Mayer, Huxley, Dobzhansky, Simpson and Haeckel explained Natural Selection in the light of Post-Darwinian discoveries. According to this theory gene mutations, chromosomal mutations, genetic recombinations, natural selection and reproductive isolation are the fie basic factors involved in the process of organic evolution.

(i) Gene mutation

Refers to the changes in the structure of the gene. It is also called gene/point mutation. It alters the phenotype of an organism and produces variations in their offspring.

(ii) Chromosomal mutation

Refers to the changes in the structure of chromosomes due to deletion, addition, duplication, inversion or translocation. This too alters the phenotype of an organism and produces variations in their offspring.

(iii) Genetic recombination

Is due to crossing over of genes during meiosis. This brings about genetic variations in the individuals of the same species and leads to heritable variations.

(iv) Natural selection

Does not produce any genetic variations but once such variations occur it favours some genetic changes while rejecting others (driving force of evolution).

(v) Reproductive Isolation

Helps in preventing interbreeding between related organisms.

Evolution by anthropogenic sources

Natural Selection (Industrial melanism)

Natural selection can be explained clearly through industrial melanism. Industrial melanism is a classical case of Natural selection exhibited by the peppered month, Biston betularia. These were available in two colours, white and black. Before industrialization peppered moth both white and black coloured were common in England. Pre-industrialization witnessed white coloured background of the wall of the buildings hence the white coloured months escaped from their predators.

Post industrialization, the tree trunks became dark due to smoke and soot let out from the industries. The black moths camouflaged on the dark bark of the trees and the white moths were easily identified by their predators. Hence the dark coloured month population was selected and their number increased when compared to the white months. Nature offered positive selection pressure to the black coloured months. The above proof shows that in a population, organisms that can adapt will survive and produce more progenies
resulting in increase in population through natural selection.

Artificial selection is a byproduct of human exploitation of forests, oceans and fisheries or the use of pesticides, herbicides or drugs. For hundreds of years humans have selected various types of dogs, all of which are variants of the single species of dog. If human beings can produce new varieties in short period, then “nature” with its vast resources and long duration can easily produce new species by selection.

Adaptive Radiation

The evolutionary process which produces new species diverged from a single ancestral form becomes adapted to newly invaded habitats is called adaptive radiation. Adaptive radiations are best exemplified in closely related groups that have evolved in relatively short time.

Darwin’s finches and Australian marsupials are best examples for adaptive radiation. When more than one adaptive radiation occurs in an isolated geographical area, having the same structural and functional similarity is referred to as convergent evolution.

Darwin’s fiches

Their common ancestor arrived on the Galapagos about 2 million years ago. During that time, Darwin’s finches have evolved into 14 recognized species differing in body size, beak shape and feeding behavior. Changes in the size and form of the beak have enabled different species to utilize different food resources such as insects, seeds, nectar from cactus flowers and blood from iguanas, all driven by Natural selection. Fig. 6.5 represents some of the finches observed by Darwin.

Genetic variation in the ALX1 gene in the DNA of Darwin finches is associated with variation in the beak shape. Mild mutation in the ALX1 gene leads to phenotypic change in the shape of the beak of the Darwin finches. Marsupials in Australia and placental mammals in North America are two subclasses of mammals they have adapted in similar way to a particular food resource, locomotory skill or climate.

They were separated from the common ancestor more than 100 million year ago and each lineage continued to evolve independently. Despite temporal and geographical separation, marsupials in Australia and placental mammals in North America have produced varieties of species living in similar habitats with similar ways of life. Their overall resemblance in shape, locomotory mode, feeding and foraging are superimposed upon different modes of reproduction. This feature reflects their distinctive evolutionary relationships.

Over 200 species of marsupials live in Australia along with many fewer species of placental mammals. The marsupials have undergone adaptive radiation to occupy the diverse habitats in Australia, just as the placental mammals have radiated across North America.

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Evidences For Biological Evolution

Paleontological evidences

Paleontology is the study of prehistoric life through fossils. Fossils are described as the true witnesses of evolution or documents of various geological strata of evolution. Fossilization is the process by which plant and animal remains are preserved in sedimentary rocks. They fall under three main categories.

(i) Actual remains:

The original hard parts such as bones, teeth or shells are preserved as such in the earth’s atmosphere. This is the most common method of fossilization. When marine animals die, their hard parts such as bones, shells, etc., are covered with sediments and are protected from further deterioration. They get preserved as such as they are preserved in vast ocean; the salinity in them prevents decay.

The sediments become hardened to form definite layers or strata. For example, Woolly Mammoth that lived 22 thousand years ago were preserved in the frozen coast of Siberia as such. Several human beings and animals living in the ancient city of Pompeii were preserved intact by volcanic ash which gushed out from Mount Vesuvius.

(ii) Petrifaction:

When animals die the original portion of their body may be replaced molecule for molecule by minerals and the original substance being lost through disintegration. This method of fossilization is called petrifaction. The principle minerals involved in this type fossilization are iron pyrites, silica, calcium carbonate and bicarbonates of calcium and magnesium.

(iii) Natural moulds and casts:

Even after disintegration, the body of an animal might leave indelible impression on the soft mud which later becomes hardened into stones. Such impressions are called moulds. The cavities of the moulds may get filled up by hard minerals and get fossilized, which are called casts.

Hardened faecal matter termed as coprolites occur as tiny pellets. Analysis of the coprolites enables us to understand the nature of diet the pre-historic animals thrived on.

Evidences from comparative anatomy

Similarities in structure between groups of organisms are accepted as indicators of relationship. For example, a comparative study of the forelimbs of different vertebrates exhibits a fundamental plan of similarity in structure. These relationships can be studied under homologous organs, analogous organs, vestigial organs, connecting links and atavistic organs.

Homologous structures

In vertebrates, comparative anatomical studies reveal a basic plan in various structures such as fore limbs and hind limbs. Fore limbs of vertebrates exhibit anatomical similarity with each other and is made of similar bones such as humerus, radius, ulna, carpals, metacarpals and phalanges.

Structures which are similar in origin but perform different functions are called homologous structures that brings about divergent evolution (Fig 6.2).

Similarly the thorn of Bougainvillea and the tendrils of Curcurbita and Pisum sativum represent homology. The thorn in former is used as a defence mechanism from grazing animals and the tendrils of latter is used as a support for climbing.

Analogous structures

Organs having different structural patterns but similar function are termed as analogous structures. For example, the wings of birds and insects are different structurally but perform the same function of flight that brings about convergent evolution (Fig. 6.3).

Other examples of analogous organs include the eyes of the Octopus and of mammals and the flippers of Penguins and Dolphins. Root modification in sweet potato and stem modification in potato are considered as analogous organs. Both of these plants have a common function of storage of food.

Vestigial organs

Structures that are of no use to the possessor, and are not necessary for their existence are called vestigial organs. Vestigial organs may be considered as remnants of structures which were well developed and functional in the ancestors, but disappeared in course of evolution due to their nonutilization. Human appendix is the remnant of caecum which is functional in the digestive tract of herbivorous animals like rabbit.

Cellulose digestion takes place in the caecum of these animals. Due to change in the diet containing less cellulose, caecum in human became functionless and is reduced to a vermiform appendix, which is vestigial. Other examples of vestigial organs in human beings include coccyx, wisdom teeth, ear muscles, body hair, mammae in male, nictitating membrane of the eye, etc.

Connecting link

The organisms which possess the characters of two different groups (transitional stage) are called connecting links. Example Peripatus (connecting link between Annelida and Arthropoda), Archeopteryx (connecting link between Reptiles and Aves).

Atavistic organs

Sudden appearance of vestigial organs in highly evolved organisms is called atavistic organs. Example, presence of tail in a human baby is an atavistic organ.

Embryological evidences

Embryology deals with the study of the development of individual from the egg to the adult stage. A detailed study of the embryonic development of different forms makes us to think that there is a close resemblance during development.

The development of heart in all vertebrates follows the same pattern of development as a pair of tubular structures that later develop into two chambered heart in fishes, three chambered in amphibians and in most reptiles and four chambered in crocodiles, birds and mammals; indicating a common ancestry for all the vertebrates, Hence scientists in the 19th century concluded that higher animals during their embryonic development pass through stages of lower animals (ancestors). Ernst Von Haeckel, propounded the “biogenetic law or theory of recapitulation” which states that the life history of an individual (ontogeny) briefly repeats or recapitulates the evolutionary history of the race (phylogeny).

In other words “Ontogeny recapitulates Phylogeny”. The embryonic stages of a higher animal resemble the adult stage of its ancestors. Appearance of pharyngeal gill slits, yolk sac and the appearance of tail in human embryos are some of the examples (Fig. 6.4). The biogenetic law is not universal and it is now thought that animals do not recapitulate the adult stage of any ancestors. The human embryo recapitulates the embryonic history and not the adult history of the organisms.

The comparative study of the embryo of different animals shows structural similarities among themselves. The embryos of fish, salamander, tortoise, chick and human start life as a single cell, the zygote, and undergo cleavage to produce the blastula, change to gastrula and are triploblastic. This indicates that all the above said animals have evolved from a common ancestor.

Molecular evidences

Molecular evolution is the process of change in the sequence composition of molecules such as DNA, RNA and proteins across generations. It uses principles of evolutionary biology and population genetics to explain patterns in the changes of molecules.

One of the most useful advancement in the development of molecular biology is proteins and other molecules that control life processes are conserved among species. A slight change that occurs over time in these conserved molecules (DNA, RNA and protein) are often called molecular clocks. Molecules that have been used to study evolution are cytochrome c (respiratory pathway) and rRNA (protein synthesis).

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Biological Evolution

Formation of protobionts

Abiotically produced molecules can spontaneously self assemble into droplets that enclose a watery solution and maintain a chemical environment different from their surroundings. Scientists call these spheres as ‘protobionts’. Liposomes are lipids in a solution that can self assemble into a lipid bilayer. Some of the proteins inside the liposomes acquired the properties of enzymes resulting in fast multiplication of molecules.

The coacervates with nucleoprotein and nutrients had a limiting surface membrane that had the characters of a virus or free living genes. Sub sequently number of genes united to form ‘proto viruses’ somewhat similar to present day viruses. Two major cell types that appeared during this time were significant. One form of the earliest cell contained clumps of nucleoproteins embedded in the cell substance.

Such cells were similar to the Monera. They are considered as ancestral to the modern bacteria and blue green algae. The other form of earliest cells contained nucleoprotein clumps that condensed into a central mass surrounded by a thin membrane.

This membrane separated nucleoproteins from the cell substances. Such cells were referred to as Protista. When the natural sources of food in the ocean declined in course of time the ancestors of Monera and Protista had to evolve different methods for food procurement. These may be summarized as parasitism, saprophytism, predator or animalism and chemosynthesis or photosynthesis. When the number of photosynthetic organisms increased there was an increase in the free O₂ in the sea and atmosphere.

CH₄ + 2O₂ → CO₂ + 2H₂O
4NH₃ + 3O₂ → 2N₂ + 6H₂O

The atmospheric oxygen combined with methane and ammonia to form CO₂ and free nitrogen. The presence of the free O₂ brought about the evolution of aerobic respiration which could yield large amounts of energy by oxidation of food stuffs. Thus Prokaryotes and Eukaryotes evolved.

Experimental approach to the origin of life

Urey and Miller (1953), paved way for understanding the possible synthesis of organic compounds that led to the appearance of living organisms is depicted in the Fig. 6.1. In their experiment, a mixture of gases was allowed to circulate over electric discharge from an tungsten electrode. A small flask was kept boiling and the steam emanating from it was made to mix with the mixture of gases (ammonia, methane and hydrogen) in the large chamber that was connected to the boiling water.

The steam condensed to form water which ran down the ‘U’ tube. Experiment was conducted continuously for a week and the liquid was analysed. Glycine, alanine, beta alanine and aspartic acid were identified. This Miller’s experiments had an insight as to the possibility of abiogenetic synthesis of large amount of variety of organic compounds in nature from a mixture of sample gases in which the only source of carbon was methane. Later in similar experiments, formation of all types of amino acids, and nitrogen bases were noticed.

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Geological Time Scale

The duration of the earth’s history has been divided into eras that include the Paleozoic, Mesozoic, and Cenozoic. Recent eras are further divided into periods, which are split into epochs. The geological time scale with the duration of the eras and periods with the dominant forms of life is shown in Table 6.1.

The Paleozoic era is characterized by abundance of fossils of marine invertebrates. Towards the later half, other vertebrates (marine and terrestrial) except birds and mammals appeared.

The six periods of Paleozoic era in order from oldest to the youngest are Cambrian (Age of invertebrates), Ordovician (fresh water fihes, Ostracoderms, various types of Molluscs), Silurian (origin of fishes), Devonian (Age of fihes, many types of fishes such as lung fishes, lobe fined fishes and ray fined fihes), Mississippian (earliest amphibians, Echinoderms), Pennsylvanian (earliest reptiles), Permian (mammal like reptiles).

Mesozoic era (dominance of reptiles) called the Golden age of reptiles, is divided into three periods namely, Triassic (origin of egg laying mammals), Jurassic (Dinosaurs were dominant on the earth, fossil bird – Archaeopteryx) and Cretaceous (extinction of toothed birds and dinosaurs, emergence of modern birds).
Cenozoic era (Age of mammals) is sub divided into two periods namely Tertiary and Quaternary.

Tertiary period is characterized by abundant mammalian fauna. This period is subdivided into five epochs namely, Paleocene (placental mammals, Eocene (Monotremes except duck billed Platypus and Echidna, hoofed mammals and carnivores), Oligocene (higher placental mammals appeared), Miocene (origin of first man like apes) and Pliocene (origin of man from man like apes).

Quaternary period witnessed decline of mammals and beginning of human social life. The age of fossils can be determined using two methods namely, relative dating and absolute dating. Relative dating is used to determine a fossil by comparing it to similar rocks and fossils of known age. Absolute dating is used to determine the precise age of a fossil by using radiometric dating to measure the decay of isotopes.

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Origin Of Life – Evolution Of Life Forms

Theory of special creation states that life was created by a supernatural power, respectfully referred to as “God”. According to Hinduism, Lord Brahma created the Earth. Christianity, Islam and most religions believe that God created the universe, the plants and the animals.

According to the theory of spontaneous generation or Abiogenesis, living organisms originated from non-living materials and occurred through stepwise chemical and molecular evolution over millions of years. Thomas Huxley coined the term abiogenesis.

Big bang theory explains the origin of universe as a singular huge explosion in physical terms. The primitive earth had no proper atmosphere, but consisted of ammonia, methane, hydrogen and water vapour. The temperature of the earth was extremely high.

UV rays from the sun split up water molecules into hydrogen and oxygen. Gradually the temperature cooled and the water vapour condensed to form rain. Rain water filled all the depressions to form water bodies. Ammonia and methane in the atmosphere combined with oxygen to form carbon-dioxide and other gases.

According to the theory of biogenesis life arose from pre-existing life. The term biogenesis also refers to the biochemical process of production of living organisms This term was coined by Henry Bastian.

According to the theory of chemical evolution primitive organisms in the primordial environment of the earth evolved spontaneously from inorganic substances and physical forces such, as lightning, UV radiations, volcanic activities, etc.,., Oparin (1924) suggested that the organic compounds could have undergone a series of reactions leading to more complex molecules. He proposed that the molecules formed colloidal aggregates or ‘coacervates’ in an aqueous environment.

The coacervates were able to absorb and assimilate organic compounds from the environment. Haldane (1929) proposed that the primordial sea served as a vast chemical laboratory powered by solar energy. The atmosphere was oxygen free and the combination of CO₂, NH₃ and UV radiations gave rise to organic compounds. The sea became a ‘hot’ dilute soup containing large populations of organic monomers and polymers.

They envisaged that groups of monomers and polymers acquired lipid membranes and further developed into the first living cell. Haldane coined the term prebiotic soup and this became the powerful symbol of the Oparin-Haldane view on the origin of life (1924-1929). Oparin and Haldane independently suggested that if the primitive atmosphere was reducing and if there was appropriate supply of energy such as lightning or UV light then a wide range of organic compounds can be synthesized.